1. Field of the Invention
This invention generally relates to a point-of-sale system including a terminal such as a cash register and an infinitely adjustable optical scanner, particularly for multidirectionally scanning a light-reflecting target, such as a bar code symbol, and/or the field of view of a detector and, still more particularly, to an arrangement for, and a method of, generating and positioning different scan patterns over a working surface of a workstation through which randomly-oriented bar code symbols pass over the working surface.
2. Description of Related Art
Targets having indicia of different light reflectivity, such as bar code symbols, have been scanned by directing a laser beam along an optical path to a symbol located in the vicinity of a reference plane lying generally normal to the optical path. The laser light reflected off the symbol is detected by a detector having a finite field of view. The symbols and/or the field of view have been scanned with many different scanning patterns in the reference plane. The pattern has been a single scan line, or a set of generally parallel scan lines extending in one direction lengthwise along the symbol. See U.S. Pat. Nos. 4,251,798 or 4,387,297. The pattern has also been two sets of intersecting parallel lines extending in two directions relative to the symbol. See U.S. Pat. No. 4,369,361.
In the case of randomly-oriented symbols, it has been proposed to produce a dense scanning pattern extending in multiple scanning directions in order to maximize the probability that the symbol will be quickly read, no matter what its orientation. Thus, conventional point-of-transaction or point-of-sale (POS) scanners, of the type generally found in supermarkets that have been built into and underneath check-out counters, deflect a laser beam in several directions and read symbols oriented in different directions that move across the reference plane, i.e. in the plane of or slightly above the countertop. To deflect the laser beam, a central mirrored polygon surrounded by a ring of many auxiliary mirrors is mounted below the countertop, each auxiliary mirror corresponding to a different scan line. Such constructions occupy a great deal of space and are easily accommodated under a supermarket countertop where a large amount of space is readily available. See U.S. Pat. No. 3,978,317.
It has also been proposed to generate a Lissajous scanning pattern in the reference plane. A pair of scanning elements, one deflecting a laser beam in the X-direction, and the other deflecting the laser beam in the Y-direction, are sinusoidally driven simultaneously to form a closed curved scan pattern. Although curved, the Lissajous pattern includes an interior central portion characterized by generally linear scan lines essentially orthogonally intersecting each other to form an X shape, and exterior portions characterized by sharply curved scan lines having small radii of curvature. Since such lightly curved scan lines are not useful for symbol reading, they are typically cut off, usually by sizing the exit window through which the outgoing laser beam passes en route to a symbol so as to permit passage therethrough of only the interior central portion of the Lissajous pattern, but to block the exterior portions thereof.
In order to generate truly multidirectional scan patterns, the use of holographic scanners was proposed in an article entitled "Multidirectional Holographic Scanner for Point-Of-Sale Bar-Code Symbol Reader", published in Optical Engineering, Nov.-Dec. 1984, Vol. 23, No. 6, p. 784ff. A holographic disc was rotated at one speed, and a laser beam incident on the disc was reversely rotated at another speed. The resultant outgoing beam had multidirectional scan lines. However, in the context of designing bar code symbol readers which are hand-held, or mounted on a support surface, where the available space is at a premium, and where it is desired to keep the overall size, weight and volume of the reader to a minimum, holographic discs are not practical. The holographic disc of said article is 200 mm in diameter--too big to provide the compactness desired in many applications for portable scanners; has an optical path of 350 mm--too long for many applications; and has a scan angle of .+-.10.degree.--too inadequate to achieve the compactness and the length of the scan desired in many applications.
The above-described scan patterns were produced by scan pattern generators comprised of electrical and optical components mounted in housings of various shapes, e.g. a gun shaped head (see U.S. Pat. No. 4,251,798 or 4,409,470), or a box-like head (see U.S. Pat. No. 4,369,361). The gun-shaped head was held in an operator's hand, and was typically connected by a cable to a remote terminal containing additional components which together comprised a laser scanning system. The terminal could either be hand-held, carried by a harness worn by the operator, or stationarily mounted, e.g. either as a stand-alone unit or within a cash register. The box-like head was also typically connected by a cable to such a remote terminal, and was supported above a countertop or like support surface to constitute a stand-alone, desk-top workstation through which objects bearing symbols passed.
Although the hand-held heads were readily movable relative to the symbols to be read, the workstation heads were more limited in terms of their freedom of movement. It was proposed to pivot the workstation heads about fixed pivots, and to move the heads linearly along a fixed path toward and away from the symbols. However, there were times when a symbol did not, or at least not readily, register with the scan pattern emitted from the workstation head, in which event, the operator would attempt to pass the symbol through the workstation again and again, thereby decreasing reading throughput. Also, there were times when ambient light in the vicinity of the workstation head was so bright that the ambient light tended to mask the reflected laser light collected by the head, thereby again decreasing system efficiency.
Also, no provision was made for changing the scan pattern. Different symbols might be more readily read by different scan patterns.